1. Field of the Invention
The present invention relates generally to optical communications systems and, more particularly, to an optical fiber amplifier disposed between an optical transmission block and an optical receiving block for amplifying signals.
2. Description of the Related Art
To keep abreast of the increasing demand for more data, wavelength-division-multiplexing (WDM) optical communication systems have been deployed to meet the increased transmission capacity. In WDM systems, the transmission capacity may be expanded by increasing the number of transmission channels or increasing transmission speed. The transmission speed required in the optical transmission has exponentially grown from 2.5 Gb/s to 10 Gb/s, and it is expected to increase more by newer developments. However, when the transmission speed is greater than 10 Gb/s, the occurrence of dispersion creates serious problems. To this end, a dispersion-compensating fiber (DCF) has been introduced to compensate the dispersion generated during the data transmission. When incorporating the dispersion-compensating fiber, it is necessary to amplify optical signals during transmission in order to compensate for the power loss of optical signals.
FIG. 1 is a simplified block diagram illustrating the configuration of a known dispersion-compensating, optical fiber amplifier. As shown in the drawing, the optical fiber amplifier includes a first through fourth isolators 120, 160, 180, and 220; a first and second pumping light sources 140 and 210; a first and second wavelength selective couplers 130 and 200; a first and second erbium-doped fibers 150 and 190; and, a dispersion-compensating optical fiber 170.
In operation, the first isolator 120 passes optical signals inputted in the optical fiber amplifier but blocks (or prevents) backwards-inputted light—for example, the light from the first wavelength selective coupler 130. The first wave selective coupler 130 couples optical signals from the first isolator 120 with the first pumped light, then outputs them to the first erbium-doped fiber 150. The first pumping light source 140 pumps the first erbium-doped fiber 150. For the first pumping light source, a laser diode can be used.
The first erbium-doped fiber 150 is pumped by the pumping light that is outputted from the first wavelength selective coupler 130, then outputs the amplified optical signals. The second isolator 160 passes optical signals that are inputted through the first erbium-doped fiber 150 while blocking any backwards-inputted light. The dispersion-compensating optical fiber 170 compensates the optical signal output from the second isolator 160. The length of the dispersion-compensating, optical fiber 170 is determined in consideration of the transmission distance of the optical signals. Normally, as the transmission distance of the optical signals is increased the degree of dispersion of the optical signals becomes severe. The third isolator 180 passes the optical signal output from the dispersion-compensating fiber 170 and blocks any backwards-inputted light.
The second erbium-doped fiber 190 is pumped by the pumping light that is inputted through the second wavelength selective coupler 200 and amplifies the optical signal output from the third isolator 180, then outputs the amplified optical signals. Particularly, the second erbium-doped fiber 190 amplifies the optical signals whose strength has been reduced while passing through the dispersion-compensating fiber 170. As such, the second wavelength selective coupler 200 couples the optical signal output from the third isolator 180 with the pumping light from the second pumping light source 210 and outputs them to the second erbium-doped fiber 190. Finally, the fourth isolator 220 passes the optical signal output from the second wavelength selective coupler 200 and blocks any backwards-inputted light.
As described above, the dispersion-compensating, optical fiber amplifier in the related art increases the total production costs as it uses a highly-priced lengthy dispersion-compensating fiber. Moreover, due to additional power loss of the optical signals, additional amplifying components, such as an erbium-doped fiber, pumping light, or wavelength selective coupler, are needed. Furthermore, to apply the dispersion-compensating amplifier to broadband high-density WDM systems, the amplifier must have a broad and flexible gain bandwidth, small noise figure, and a plurality of isolators for those purposes. This consequently makes the designing process much more complicated and by adding more components the entire production costs of the optical fiber amplifier is increased as well as the volume of the amplifier.